Field Guide. cropping and pastoral grazing on flat to rolling country.

SOIL INDICATORSSusceptibility to wind <strong>and</strong> water erosion under <strong>cropping</strong> Assess, based on your knowledge of the area or visual observationsduring the season, whether the amount of wind erosion during <strong>and</strong>after cultivation has become a concern. Take into account the size of the dust plume or clouds raised bycultivation, <strong>and</strong> whether the material stays within the paddock,within the farm, or is blown into the surrounding district (seeopposite). Determine the severity of water erosion by augering or digging holesto compare the difference in topsoil depths between the crest <strong>and</strong>footslope, <strong>and</strong> by observing the amount of rill <strong>and</strong> sheet erosion,<strong>and</strong> sedimentation into adjacent drains <strong>and</strong> streams.THE SUSCEPTIBILITY OF A SOIL TO WIND EROSION depends onfactors including soil moisture, <strong>and</strong> wind velocity, surface roughness<strong>and</strong> particle size. Soils that have low amounts of organic matter <strong>and</strong>have lost their soil structure through compaction <strong>and</strong> over-cultivationare pulverised to dust on further cultivation, making them vulnerableto wind erosion if unprotected. Wind erosion reduces the productivepotential of soils through nutrient losses, lower available water-holdingcapacity, <strong>and</strong> reduced rooting volume <strong>and</strong> depth. Research has shownthat an extra 25 mm of water stored in the soil has the potential toincrease maize yields by approximately 600 kg/ha. Wind-blown materialdeposited after a strong north-westerly wind in north Canterbury hasbeen shown to contain high levels of Ca, K, Mg, <strong>and</strong> N. Available Plevels present were equivalent to a loss of 73 kg of superphosphateper hectare. The water erodibility of soil on sloping ground is governedby factors including the amount <strong>and</strong> intensity of rainfall, <strong>and</strong> the soilinfiltration rate <strong>and</strong> permeability. The latter two are governed by soilstructure.30 VISUAL SOIL ASSESSMENT: Volume 1